1. Field
Embodiments of the present invention relate to a particle image velocimetry system for a three-dimensional space, the system involving illuminating tracer particles flowing around an object with laser light at two times separated by a very small period of time, taking images of the tracer particles illuminated with the laser light by an imaging device, and measuring the velocity field of three velocity components in the three-dimensional space by tomography from the obtained images at the two times.
2. Description of the Related Art
PIV (Particle Image Velocimetry), in which fluid including tracer particles is illuminated with sheet-shaped illumination two times with a predetermined interval therebetween, images of the tracer particles illuminated with the illumination are taken by one imaging means, and the amount of movement of the tracer particles is determined from the images at the two times to thus measure a two-dimensional distribution of the flow velocity of the fluid is known from Japanese Patent Application Laid-open No. 2007-85784 and Japanese Patent Application Laid-open No. 2011-180028.
Furthermore, tracer particles included in an unsteady flow flowing around an object are illuminated with volume illumination having a rectangular, etc. shape at two times with a very short time interval therebetween, and an image of the illuminated tracer particles is taken by imaging means in at least three directions. A three-dimensional image is reconstructed for each of the images taken at time 1 and time 2 using tomographic theory. A particle image velocimetry system employing tomographic PIV that measures the velocity field of three velocity components of a tracer particle group from a cross-correlation between the three-dimensional images at the respective times is known from Non-Patent Document 1 (6th International Symposium on Particle Image Velocimetry Pasadena, Calif., USA, Sep. 21-23, 2005, “Tomographic particle image velocimetry” G. E. Elsinga, F. Scarano, B. Wieneke, B. W. van Oudheusden).
Meanwhile, the arrangement described in Non-Patent Document 1 above has the problem that, since images of the region illuminated with volume illumination are taken from different directions by means of a plurality of cameras, when the position of tracer particles in three-dimensional space is calculated from the obtained images, a tracer particle (ghost particle) that is not actually present is recognized as if it were present.
FIGS. 7A and 7B schematically show states in which images of two tracer particles T1 and T2 are taken by two cameras C1 and C2; it is assumed that as shown in FIG. 7A the tracer particles T1 and T2 appear in pixels a and b of the camera C1, and at the same time the tracer particles T1 and T2 appear in pixels c and d of the camera C2. As shown in FIG. 7B, the position of the tracer particle obtained from information of the pixels a and c and the position of the tracer particle obtained from information of the pixels b and d correspond to those of the tracer particles T1 and T2, which are actually present, and the position of the tracer particle obtained from information of the pixels a and d and the position of the tracer particle obtained from information of the pixels b and c include, in addition to the tracer particles T1 and T2, ghost particles T3 and T4, which are not actually present. Due to the positional information of tracer particles T1 and T2, which are actually present, being contaminated with the positional information of the ghost particles T3 and T4, the measurement precision of the three-dimensional velocity field is degraded
In order to exclude information regarding ghost particles and leave only information regarding tracer particles that are actually present, the number of cameras may be increased. FIG. 8 shows the relationship between the number of cameras and the ghost ratio (number of actual tracer particles/number of ghost particles); for example, in order to achieve a ghost ratio of 1, it is necessary to employ 12 cameras. However, increasing the number of cameras not only causes the equipment cost to increase greatly but also makes it difficult to ensure that there is a space for installing the cameras and, moreover, there is a problem that it requires much time and manpower for setting up the cameras.